Field of the Invention
The present invention relates to an automatic transmission for a vehicle.
Description of Related Art
In the field of an automatic transmission, more multiplicity of shift-stages is useful technology for enhancement of fuel consumption and drivability of a vehicle.
In this sense, research for an engine has been made to achieve weight reduction and to enhance fuel consumption by so-called downsizing and research on an automatic transmission has been performed to simultaneously provide better drivability and fuel consumption by achieving more shift stages.
To achieve more shift-stages for an automatic transmission, the number of parts is typically increased, which may deteriorate installability, a production cost, weight and/or power flow efficiency.
Therefore, to maximally enhance fuel consumption of an automatic transmission having more shift-stages, it is important that better efficiency is derived by less number of parts.
In this background, an eight-speed automatic transmission has been introduced recently and a planetary gear train for an automatic transmission facilitating more shift-stages is under investigation.
In addition, a recent eight-speed automatic transmission typically shows a gear ratio span in a level of 6.5 to 7.5, which may require improvement for better fuel consumption.
In the case of a gear ratio span of an eight-speed automatic transmission having a level above 9.0, it is difficult to maintain step ratios between adjacent shift stages to be linear, by which driving efficiency of an engine and drivability of a vehicle deteriorated. Thus, research studies are underway for developing a high efficiency automatic transmission having nine or more speeds.
The information included in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and may not be taken as an acknowledgment or any form of suggestion that this information forms the prior art already known to a person skilled in the art.